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          <h1 class="post-title" itemprop="name headline">iOS后台实践</h1>
        

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        <p>一提到iOS后台机制，网上搜了下一大堆资料，众说纷纭，现结合<a href="https://developer.apple.com/library/content/documentation/iPhone/Conceptual/iPhoneOSProgrammingGuide/BackgroundExecution/BackgroundExecution.html" target="_blank" rel="noopener">苹果官方文档</a>及自身实际对现阶段(iOS10)iOS后台技术做个梳理。APP应用状态分为激活、未激活、后台:</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line">typedef NS_ENUM(NSInteger, UIApplicationState) &#123;</span><br><span class="line">    UIApplicationStateActive, // 前台</span><br><span class="line">    UIApplicationStateInactive, //后台未在运行</span><br><span class="line">    UIApplicationStateBackground //后台运行</span><br><span class="line">&#125; NS_ENUM_AVAILABLE_IOS(4_0);</span><br></pre></td></tr></table></figure>
<p>本文要讨论就是UIApplicationStateBackground模式。Apple提供了以下三种方式：</p>
<ul>
<li>方式一：延长小段时间(beginBackgroundTaskWithName)</li>
<li>方式二：系统托管下载任务(NSURLSession)</li>
<li>方式三：特定类型APP（Audio、Location、VOIP、Bluetooth、Newsstand、External accessory、Background fetch、Remote notifications）</li>
</ul>
<h3 id="方式一：延长小段时间-beginBackgroundTaskWithName"><a href="#方式一：延长小段时间-beginBackgroundTaskWithName" class="headerlink" title="方式一：延长小段时间(beginBackgroundTaskWithName)"></a>方式一：延长小段时间(beginBackgroundTaskWithName)</h3><p>为了保证某些短任务能够顺序执行完成，iOS允许APP向系统申请一小段时间以供任务顺利完成，代码如下</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br></pre></td><td class="code"><pre><span class="line">- (void)applicationDidEnterBackground:(UIApplication *)application</span><br><span class="line">&#123;</span><br><span class="line">    bgTask = [application beginBackgroundTaskWithName:@&quot;MyTask&quot; expirationHandler:^&#123;</span><br><span class="line">    	//系统即将进入inactive</span><br><span class="line">        [application endBackgroundTask:bgTask];</span><br><span class="line">        bgTask = UIBackgroundTaskInvalid;</span><br><span class="line">    &#125;];</span><br><span class="line"> </span><br><span class="line">	//主动结束</span><br><span class="line">    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^&#123; </span><br><span class="line">        [application endBackgroundTask:bgTask];</span><br><span class="line">        bgTask = UIBackgroundTaskInvalid;</span><br><span class="line">    &#125;);</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>PS：beginBackgroundTaskWithName向系统申请延长休眠时间，endBackgroundTask表示系统可以进入休眠</p>
<p>需要注意的是：查了网上其他文章，都说可以最长申请600s的时间，但实际测试只有179s左右（应该跟系统有关，iOS6是600s，iOS7以上是180s，我的系统是iOS 10.0.1），可以通过backgroundTimeRemaining查询（打印是179.988913s），但没有找到可以设置的地方。<strong>需要注意的是backgroundTimeRemaining只在进入后台是有效，前台的话会是一个很大的值</strong></p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">@property(nonatomic,readonly) NSTimeInterval backgroundTimeRemaining NS_AVAILABLE_IOS(4_0);</span><br></pre></td></tr></table></figure>
<h3 id="方式二：系统托管下载任务-NSURLSession"><a href="#方式二：系统托管下载任务-NSURLSession" class="headerlink" title="方式二：系统托管下载任务(NSURLSession)"></a>方式二：系统托管下载任务(NSURLSession)</h3><p>待补充</p>
<p>参考：</p>
<ul>
<li><a href="https://onevcat.com/2013/08/ios7-background-multitask/" target="_blank" rel="noopener">WWDC 2013 Session笔记 - iOS7中的多任务</a></li>
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              <div class="post-toc-content"><ol class="nav"><li class="nav-item nav-level-3"><a class="nav-link" href="#方式一：延长小段时间-beginBackgroundTaskWithName"><span class="nav-number">1.</span> <span class="nav-text">方式一：延长小段时间(beginBackgroundTaskWithName)</span></a></li><li class="nav-item nav-level-3"><a class="nav-link" href="#方式二：系统托管下载任务-NSURLSession"><span class="nav-number">2.</span> <span class="nav-text">方式二：系统托管下载任务(NSURLSession)</span></a></li></ol></div>
            

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